I did not read all the replies so Im sorry if this is just a repeat of advice but increased stress can cause lighter periods or for you to even totally miss a period. Also heavy exercise can do the same. This may sound strange but often times woman who are around eachother a lot or live together have cycles that are either at the same time as eachother or just before or after...our bodies do that on purpose. Kind of funny but true. I have had mine change when living with my sister in law or my sister. Beyond that I wouldnt know exactly what to suggest but go to the doctor and Im sure you will find out it is nothing to worry about.

Best Wishes!